1. Field of the Invention
This invention relates to a composite tube for use in heat exchangers.
2. Disclosure Information
Designers of heat exchangers must consider a number of interconnected performance factors. For example, high heat transfer efficiency is promoted by turbulent flow within the fluid conducting tubes of the heat exchanger. However, such turbulence is usually achieved only at the cost of a high pumping loss, which may necessitate the use of considerable power to move fluid through the heat exchanger device. If, on the other hand, fluid is allowed to flow through the heat exchanging tubes in a quiescent manner, stratification will occur and the efficiency of the unit will be diminished.
Heat exchanger designers have employed a variety of methods for increasing turbulent flow through the tubes of heat exchangers. One familiar technique involves the insertion of an additional element into the tube so as to promote turbulent flow. U.S. Pat. Nos. 2,096,272; 2,480,706 and 3,734,135 all disclose inserted turbulence promoting devices. French Pat. No. 702,989 and British Patent Specification No. 1,016,573 disclose additional types of inserted turbulence promoting elements. Each of the cited examples of turbulence promoting elements suffers from a common drawback inasmuch as additional expense in the form of both labor and material is required to insert the turbulator strips into the partially finished tube assemblies. An additional drawback resides in the fact that with certain metals such as aluminum, corrosion of the tube material is promoted by the formation of crevices at the intersections of the turbulence promoting devices and the interior surfaces of the outer tube walls. This so-called crevice or poultice corrosion is particularly bothersome with aluminum radiator tubes. It is an object of the present invention, then, to avoid the added labor costs usually associated with producing heat exchanger tubes having a turbulence promoting feature, while at the same time avoiding corrosion problems.
Heat exchanger designers have attempted to avoid drawbacks associated with separate turbulator inserts by providing integral turbulence promoting features in heat exchanger tubes. U.S. Pat. Nos. 1,881,610; 3,154,141; 3,875,997; 3,906,605 and 4,470,452 as well as French Pat. No. 1,336,583 and Japanese Pat. No. 59-41795 all disclose heat exchanger tube structures in which the tube wall is worked in order to provide a turbulence promoting feature such as a plurality of indentations formed in the wall. Such structures suffer from two potential drawbacks. First, in the event that the tube wall is of sufficient thickness so as to withstand the roughening process without resulting in any distortion of the outer surface of the tube, the heat transfer characteristics of the tube will be somewhat impaired by the thickness of the wall itself. Furthermore, the expense of the tube material will be increased because of the thicker wall. If, on the other hand, a thin wall tube is used such that the roughening turbulator structure appears not only on the inner wall of the tube but also on the outer wall of the tube, another problem results. Such a construction, featuring a plurality of indentations in the tube wall, is shown in the '452 patent, which is assigned to the assignee of the present invention, and which is hereby incorporated by reference herein. It has been determined that a turbulator radiator tube according to the '452 patent may be joined to the header tanks of the heat exchanger only with difficulty when certain brazing processes are used because the turbulence-promoting indentations in the tube wall are difficult to fill with brazing material, and leaks result in the finished heat exchanger. This necessitates an additional sealing process, which materially increases the cost of the heat exchanger.
It is object of the present invention to provide a composite heat exchanger tube which obviates the need for a separate turbulence producing insert but which nevertheless provides good heat transfer characteristics.
It is yet another object of the present invention to provide a composite heat exchanger tube which may be easily processed during the assembly of a heat exchanger employing such a tube.
It is an advantage of the present invention that a tube according to the present invention will be resistant to crevice or poultice corrosion.
It is yet another advantage of the present invention that a composite heat exchanger tube according to this invention may be fabricated without additional labor costs associated with other types of turbulence promoting heat exchanger tubes.
It is yet another advantage of the present invention that a composite heat exchanger tube according to this invention will have superior mechanical strength as compared to other types of heat exchanger tubes.
It is yet another advantage of the present invention resides in the fact that a composite heat exchanger tube according to the present invention has a smooth exterior surface which is easily joined to a heat exchanger header component by conventional brazing or welding methods.
Other objects, features and advantages of the present invention will become apparent to the reader of this specification.